A pandemic can occur when a new subtype of influenza virus, not previously circulated among humans, mutates or reassorts (that is, shuffles genetic material between human and avian strains) to allow human-to-human transmission. So far, the avian flu is not contagious among humans. But a more dangerous form of the virus could conceivably evolve that would be spread like a typical seasonal flu, through the coughing and sneezing of infected individuals. The specific incubation period for such a form of avian flu would not be known until after it appeared; if it acted like other forms of the flu virus, then the incubation period would last two days. Victims might be contagious for a day before and up to five days after first experiencing symptoms.
An avian influenza virus originally resides in wild aquatic birds, but may make its way into domestic birds, including ducks and chickens. According to the United States Centers for Disease Control and Prevention (CDC), the H5N1 strain of avian influenza has caused the largest and most severe outbreaks of flu in poultry ever recorded. International health organizations have been monitoring the strain for the past eight years. The World Health Organization has confirmed 173 cases of the avian flu in humans; the majority of those infected had had some contact with diseased birds. As of March 24, 2006, more than half of these cases (97 in all) had resulted in death. Most of the deaths were in Asia.
The CDC has determined that viruses containing avian influenza genes caused both the 1957 and the 1968 pandemics. The 1918 virus also appears to have been of avian origin. Yet it is unusual for an avian virus alone to be transmitted to humans. This year, two separate groups of researchers, one in Japan, the other in Holland, reported that they had discovered why the avian flu is rarely transmitted from one person to another. They found that the avian virus tends to find its way deep into a person’s respiratory tract, keeping it from spreading through coughing and sneezing as with a “normal” flu.
This natural barrier to transmissibility is fortunate since global connectivity greatly accelerates the spread of more contagious strains. Computer models suggest that a pandemic virus that infected people with the same transmissibility as a typical flu virus could travel around the globe in as little as two weeks.
Further risk resides in the current drug pipeline. A vaccine for a human flu caused by the H5N1 virus does not yet exist because a vaccine cannot be developed for a new human-to-human flu strain until that strain is available to researchers. Vaccine development would take at least three to six months. Production facilities are not capable of making enough vaccine for the world’s population. Antiviral medications are already on the market, and preliminary tests have shown them to be generally effective against avian flu, especially at higher dosages. But there are no guarantees that existing medications will work against a new strain, that the supply will be sufficient, or that the medications will remain effective as the virus continues to mutate.
Pandemics are unique among large-scale disasters; they leave the supporting physical infrastructure of roads and buildings largely intact. Only the people who operate these lifeline infrastructures are affected. The U.S. Department of Health and Human Services estimates that a pandemic comparable to the 1918 outbreak could cause 1.9 million deaths in the United States. In that projection, there would be 90 million people ill, 1.5 million hospitalized in intensive care, and 742,000 people needing mechanical ventilators to breathe. This situation would put a huge strain on public and private health systems; for example, there are now only 105,000 mechanical ventilators in the United States. In the U.S. alone, economic losses would range between $71 and $167 billion.